Harmonic absolute instability in the ${\mathrm{TE}}_{21}$ mode is found to cause mode competition in a new millimeter-wave gyrotron traveling-wave experiment designed for high stability in the ${\mathrm{TE}}_{11}$ mode. Complete suppression of this instability by the application of the ${\mathrm{TE}}_{11}$ drive has been observed with beam current up to at least 6 times the instability threshold, which results in high amplified power (∼25 kW) at high efficiency (∼23%). The study is significant in that it demonstrates that cyclotron-maser amplifiers may operate at a current well above oscillation threshold without degradation in saturated performance.

• Received 3 April 1989

DOI:https://doi.org/10.1103/PhysRevLett.63.1062